Geostrophic approximation in the St. Lawrence estuary
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A 1965 survey of currents and geostrophic currents in the St. Lawrence estuary is described. An innovation employed in the survey was to moor the strings of oceanographic bottles in the cross-section and trip them simultaneously. A tidal oscillation was detected in the vertical shear of the geostrophic current as well as in the vertical shear of the axial and cross-channel current components. The observations qualitatively confirm predictions from a simple theory that is presented for geostrophic response on one-and two-layer canals. The theory suggests that the period of resonant cross-channel oscillation is an important time scale since current fluctuations of much longer periods reflect accurately in the geostrophic current, while fluctuations of shorter periods may appear as considerable distortions in the geostrophic current. From this, it is concluded that a single determination of geostrophic current may represent neither the instantaneous nor the long-term average current. The average geostrophic current over a time interval longer than the resonant period may, however, represent the average current over the same interval.Keywords:
Geostrophic current
Oscillation (cell signaling)
Warm water of the Tsushima Current converges southwest of the Tsugaru Strait and flows relatively stable northward with most narrow cross section in the Japan Sea. Therefore, many Japanese oceanographers have evaluated the volume transport in this region to monitor the interannual or seasonal variation of the Tsushima Current system. However, the description on the water movements by the past studies have been only inferred from the dynamic caluclation on the basis of hydrographic data. From April 1992 to March 1993, we carried out the direct current measurement southwest of the Tsugaru Strait by using of a moored instrument at the depth of 130m (Apr.∼Oct.1992) and 180m (Oct.1992∼Mar.1993) to investigate the dynamical variability of the Tsushima Current. The large baroclinic events appear to be dominated mainly by passing through the seaward meander of the coastal current in May 1992 and by merging a warm eddy with the spatial scale of several hundred kilometers within the coastal curent in January 1993. Moreover, about one month-periodical fluctuations observed in the temparature and the on-offshore current corresponded with a Tsushima Current frontal eddy event that could be identified in the NOAA/AVHRR images. From a comparison between the current velocities observed by the current meter and the geostrophic ones, it is suggested that the Tsushima Current includes the evidence as the barotoropic northward flow with about 10 cm s-1 tobe unrelated with the seasonal changes of stratification. This implies that the actual Tsushima Current transports are likely to be larger than those estimated by the past.
Current meter
Geostrophic current
Meander (mathematics)
Halocline
Stratification (seeds)
Outflow
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In order to determine the validity of applying a geostrophic relationship to the circular currents in Lake Biwa, a series of quasi-simultaneous observations of currents and water temperature were carried out. The current measurements were made using the drifters with cross-board placed at several depths. The vertical profiles of water temperature were measured by a bathythermograph at four points about 500 m away from the current station. The comparisons between the directly measured current and estimated geostrophic current show that the geostrophic approximation is not so valid for the current observed at a specified time and space. On the other hand, the current averaged over a time long enough to cancel the effect of the slowest internal wave, can serve as the baroclinic geostrophic current including the centrifugal force.
Geostrophic current
Bathythermograph
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The three-dimensional structure of western boundary current of the South China Sea(SCS) in winter is investigated with the unstructured grid,Finite-Volume Coastal Ocean Model(FVCOM) and the altimeter data.The dynamic mechanisms of the current are explored by the numerical experiment and the dynamical diagnose analysis.The modeling results and the altimeter data show that the western boundary current has greater velocity than nearby current,which comes from northern SCS and flows along the 200m isobath towards southwest.It's strengthened at the southeast of Hainan Island and forms the south rush flow.The horizontal and vertical structure of western boundary current is complicated and variable.Since the pressure gradient force is associated with the Coriolis force,the western boundary current is generally dominated by the geostrophic currents.On the other hand,the current has local characteristics because of the impact of the winds,topography and the nonlinear advection,etc.Among these factors,the pressure gradient force and winds have the most significant contributions on the formation and evolution of the western boundary current.The distribution of pressure gradient force is agreed with the three-dimensional structure of the current very well.The impact of winds on the current is related to the local topography and the coastline.
Boundary current
Pressure-gradient force
Pressure gradient
Geostrophic current
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Geostrophic current
Hydrographic survey
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Geostrophic current
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We have investigated sea level and surface currents features over the Newfoundland Shelf and Slope using the tandem TOPEX/Poseidon (T/P) and Jason altimetry data (2002–2003). The consistency and error characteristics of T/P and Jason measurements are examined not only in terms of sea level and cross‐track current anomalies but also with respect to current anomalies at crossovers and the Labrador Current transport. Nominal absolute currents are constructed by adding the altimetric geostrophic current anomalies to a climatological‐mean model circulation field. The comparison of the sea level and cross‐track current anomalies from January to July 2002 shows overall good agreement between T/P and Jason, with correlation coefficients different from zero at the 95% confidence level at almost all locations for sea level and at most locations for currents. Errors are estimated to be 2.5 cm for sea level and 10 cm/s for cross‐track current anomalies. Analyses of the current variability at crossovers indicate approximate agreement of T/P and Jason measurements, except for the Northeastern Newfoundland Shelf and Slope probably due to the ice presence during the period. Model‐altimetry combined absolute currents are used to estimate near‐surface transport associated with the shelf‐edge Labrador Current, showing good correlation between T/P and Jason estimates and strong seasonal changes. The cross‐track geostrophic current anomalies from September 2002 to December 2003 are used to calculate the root‐mean‐square (rms) current variability at crossovers and to derive the shelf‐edge Labrador Current. The interleaved T/P and Jason observations can better capture the spatial distribution of shelf and slope circulation variability.
Geostrophic current
Anomaly (physics)
Sea-surface height
Dynamic height
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Current meter
Geostrophic current
Eddy
Anticyclone
Forcing (mathematics)
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The seasonal and interannual variations of surface geostrophic current in the Southeast Pacific(5°~55°S,70°~120°W) have been analyzed using Topex/Poseidon altimeter data during 1992~2001.The results show that the South Equatorial Current is characterized with seasonal variation on the magititude of velocity,while the Peru Current and West Wind Drift reflect the shift of the current axes.The interannual variation of the current field is mainly caused by El Nino events.During the El Nino events,both South Equatorial Current and Peru Current at the low latitude region turn their directions to be eastward,and the current field at the higher latitude region is strongly weakened,with smaller current speed and random direction.
Geostrophic current
Seasonality
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Three years (2008–2011) of current measurements outside Pemba (∼13°S), Mozambique, are presented. Oil and gas reserves are found in the region, and knowledge of the ocean current climate is important for safe offshore operations. The full-depth current profile at Pemba was estimated by comparing with moorings further south. The moored record was also compared with geostrophic velocities obtained from sea surface height (SSH) fields, current profiles from ship-mounted Acoustic Doppler Current Profiler and surface currents from seven drifters passing the mooring. Velocities up to 60 cm/s were observed at 850 m depth. Altimetry-derived currents, while strong (often > 100 cm/s), are probably underestimates due to coarse resolution. Direct observations show surface currents up to 170 cm/s. Four strong current events were studied in detail. Three of the four events appeared to be associated with anticyclonic eddies nearby. The time series of SSH-derived geostrophic current always showed southward velocities, never below 20 cm/s. These findings indicate that currents off Pemba may display characteristics of a western boundary current rather than being completely dominated by eddies. Strong current events are probably caused by anticyclones interacting with the boundary current and speeds on the order of 200 cm/s can be expected.
Eddy
Boundary current
Geostrophic current
Anticyclone
Mooring
Sea-surface height
Current meter
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Altimeter data from Jason-1 satellite are very useful in providing general and continuous information about the ocean, including sea surface currents. The main objective of study is to identify the most appropriate mathematical equation for determining sea surface current in the South China Sea. The seasonal changes of surface current during different monsoon periods in 2004 and 2005 were also identified. The equations used to derive sea surface current are geostrophic current equations, wind-driven current equations and tidal current equations. The methodology of this study involves the use of sea level height and sea surface wind speed data from Jason-1 satellite altimeter to derive geostrophic current and wind-driven current. Tidal amplitudes from co-tidal charts were used to derive tidal current. The derived surface currents were used to produce combined geostrophic and wind-driven current. Combined geostrophic and tidal current as well as total surface current which is the combination of geostrophic current, wind-driven current and tidal current were also derived. Maps of total surface current circulation pattern were produced during four monsoon periods in 2004 and 2005. Regression analysis and comparison of mean and standard deviation values with sea truth data were carried out to identify the most appropriate equation of surface current for the South China Sea. Results of the analysis indicate that total surface current speed and direction have good correlation with the sea truth data, that is 0.68 and 0.70 respectively. The analysis by comparing the mean values indicate that there are no significant difference between the means of total surface current and the means of sea truth data. The standard deviations of total surface current are smaller compared to the sea truth data values. In conclusion, altimeter data from Jason-1 satellite combined with tidal data to derive the total surface current is appropriate to determine sea surface current circulation pattern in the South China Sea.
Geostrophic current
Sea-surface height
Ocean surface topography
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